<form action="do_login.php" method="GET">
User: <input id="login" name="login" type="text" /><br />
Pass: <input id="pass" name="pass" type="password" /><br />
<input type="submit" value="login" />
<?php
if (isset($_GET['error']) && $_GET['error'] == "true"){ 
?>
<div style="color:red">Credenciales Erroneos</div>
<?php } ?>
</form>